1. Field of the Invention
This invention relates to pipes made of polyethylene resins, excellent in rigidity, impact resistance and long-term durability under stress (environmental stress cracking resistance, internal pressure creep resistance).
2. Background Art
Polyethylene pipes are light in weight, easy to handle, and are non-corrosive. In addition, their rigidity is so high that they can be laid under the ground, and their flexibility is also so high that they can follow a movement of ground. Thanks to these advantageous characteristics, the amount of polyethylene pipes used is rapidly increasing in recent years.
Polyethylene pipes are required to have the following properties:
(1) the above-described characteristics; PA1 (2) impact resistance sufficient to endure impacts given at the time when and after they are set; and PA1 (3) excellent long-term durability under gas or water pressure (specifically, environmental stress cracking resistance and internal pressure creep resistance). The conventional polyethylene pipes already posess the above properties (1) and (2). With respect to the long-term durability (3), the conventional pipes can meet the ISO standard, i.e. 50-year durability at normal temperatures under an internal pressure, expressed in terms of circumferential stress, of approximately 8 MPa. However, the conventional polyethylene pipes are still insufficient in the long-term durability for use under severer conditions, such as main pipes for gases or running water which have a large diameter and undergo high internal pressure. For this reason, they are presently used only for branch pipes and the like, having a small diameter. PA1 (1) a melt flow rate of 0.02 to 0.2 g/10 min, PA1 (2) a flow ratio of 50 or more, PA1 (3) a density of 0.945 to 0.960 g/cm.sup.3, and PA1 (4) a relaxation parameter H, represented by the following equation (I), of 2.00.times.10.sup.-8 dyn/cm.sup.2 or less: ##EQU2## PA1 wherein E(.tau.) represents a relaxation modulus at time .tau.. PA1 (a) the polymerization reaction is carried out in two stages, that is, in the presence of the reaction product obtained by the polymerization carried out in the first reaction zone, the polymerization is further continued in the second reaction zone; PA1 (b) in either one of the first and second reaction zones, ethylene is homopolymerized in the presence of hydrogen, the molar ratio of the hydrogen to the ethylene contained in the gas phase being from 0.5 to 5, so as to produce polymer A in an amount of 30-70% by weight of the amount of the polymer finally produced; PA1 (c) in the other reaction zone, ethylene and the .alpha.-olefin are copolymerized in the presence of hydrogen, the molar ratio of the hydrogen to the ethylene contained in the gas phase being from 0.001 to 0.5, so as to produce polymer B in an amount of 30-70% by weight of the amount of the polymer finally produced; and PA1 (d) the polymerization conditions are so controlled that the final polymer has the previously-mentioned physical properties, that is, an MFR of 0.02 to 0.2 g/10 min, an FR of 50 to 170, a density (.rho.) of 0.945 to 0.960 g/cm.sup.3, and a relaxation parameter H of 2.00.times.10.sup.31 8 dyn/cm.sup.2 or less. PA1 (1) a melt flow rate (MFR) of 0.02 to 0.2 g/10 min, preferably 0.03 to 0.15 g/10 min, more preferably 0.03 to 0.1 g/10 min; PA1 (2) a flow ratio (FR) of 50 or more, preferably 60 or more, more preferably 75 or more; PA1 (3) a density of 0.945 to 0.960 g/cm.sup.3, preferably 0.948 to 0.960 g/cm.sup.3, more preferably 0.950 to 0.955 g/cm.sup.3 ; and PA1 (4) a relaxation parameter H, represented by the above equation (I), of 2.00.times.10.sup.-8 dyn/cm.sup.2 or less, preferably 1.95.times.10.sup.-8 dyn/cm.sup.2 or less, more preferably 1.90.times.10.sup.-8 dyn/cm.sup.2 or less.
The long-term durability of a polyethylene pipe is considered to be determined by the environmental stress cracking resistance, that is the resistance to cracking which is caused when an internal pressure applied to the pipe acts, as a tensile stress in the circumferential direction, on the pipe over a long period of time. Therefore, in order to improve the long-term durability of polyethylene pipes, it is necessary to improve the environmental (tensile) stress cracking resistance.
The environmental stress cracking resistance of the conventional polyethylene materials for pipes, as evaluated by the method described below, have been found to be at most 20 hours.
In order to improve the environmental stress cracking resistance of a polyethylene, it is known to increase the molecular weight or to decrease the density of the polyethylene. However, when the molecular weight is increased, the fluidity of the polyethylene is lowered, so that the molding properties such as pipe-extrusion properties and injection moldability are impaired. When the density is decreased, the rigidity of the polyethylene is unfavorably lowered.
An object of the present invention is to overcome the aforementioned drawbacks in the prior art and provide pipes made of polyethylene resins, which have improved environmental stress cracking resistance and thus have improved long-term durability with the rigidity and impact resistance maintained high and which can be produced with good moldability at high productivity.